PERIOD OF INCUBATION". — No fixed period of incubation can as yet be given to malaria acquired in the natural way. While in the majority of cases it would appear to average from six to fourteen or twenty days, yet instances have been re ported in which the disease developed within a few hours after exposure to the infection, and still other instances in which the evidences of infection did not occur for weeks or months after exposure. In the former class of cases, as studied by Plehn, the earliest evidences of supposed infection consisted of a single paroxysm immediately after exposure, no other paroxysms being experienced for several days subsequently. At the time of the first paroxysm examination of the blood yielded negative results, the parasite not being discovered until the paroxysms re curred, some days later. Instances of prolonged incubation are susceptible to the explanation that they are, in all probabilit3-, cases of relapses of earlier attacks that have been characterized by manifestations so mild as to be over looked.
In view of our present knowledge of malaria some deg-ree of variation in the length of incubation may readily be ac counted for by the varying periods re quired for the development of the differ ent forms of parasites. Further, inas much as the clinical manifestations of the disease begin when the parasite has developed into a group sufficiently large to produce a reaction at the time of ulation, the period of incubation will also vary in accordance with tbe number of parasites originally introduced into an individual. This partial dependence of the duration of the period of incubation upon the number of parasites producing the infection is well shown in the cases in which infection is artificially brought about by inoculation.
Inoculation-experiments as determin ing the duration of incubation of malaria have been of much interest, and the vary ing results obtained in infection by the different forms of parasites correspond to the differences noted in cases that oc cur spontaneously.
[Mannaberg makes the following de ductions from the results of his experi ments: In five cases inoculated with the quartan parasite the minimum period of incubation was 11 days, the maximum period IS days, and the mean period 13.4 days.
Seven cases inoculated with the tertian parasite showed a minimum incubation period of 6 days, a maximum of 21 days, and a mean of 11 days.
Seven eases inoculated with thc t-estivo autumnal parasite (with anueb, but without crescents) gave a minimum period of incubation of 3 days, a maxi mum of 14 days, and a mean of 6.5 days; while two cases inoculated with crescents without ("probably a few") arncebre gave an incubation period of 13 and 15 days, respectively, or a. mean of 14 days. JAAIF.S C. WILSON and TtiomAs G. Asti Tox.] These experiments indicate that the longest periods of incubation are associ ated with the milder forms of infection, and that the grave infections, the mstivo autumnal, show the shortest periods. In both instances inoculation-experi ments coincide with clinical experience, and render easy of belief the probability that in the malignant cases of testivo autumnal infection the incubation may be brief.